Seawater desalination is an important process due to increased water demand and decreased suitable water sources. With the increasing awareness of the impact that seawater contaminants have on the environment and human health, water quality standards are becoming increasingly stringent. Boron is a naturally occurring nonmetallic element that is present in seawater at an average concentration of 4.7 milligrams per liter (mg/L). Dissolved boron in seawater occurs predominantly in the boric acid form (B(OH)3), with lesser amounts of the dissociated forms (H2BO3−, HBO32−, and BO33−). While boron is a vital element for organism growth, excessive exposure can cause detrimental effects to plants, animals, and possibly humans. Consequently, the World Health Organization (WHO) has issued Guidelines for Drinking Water Quality that propose a maximum recommended boron concentration in drinking water of 2.4 mg/L. Boron also is on the U.S. Environmental Protection Agency's (EPA) Drinking Water Contaminant Candidate List, though there is limited information on the occurrence of boron in drinking water supplies in the United States.
The availability of technologies to remove boron from seawater is limited. Treatment options include ion exchange processes, such as those that include the use of ion exchange resins, such as boron-specific resins consisting of a styrene-divinylbenzene (DVB) backbone with N-methyl glucamine active sites, and resins that are a mixture of conventional strong-acid-strong-base mixed bed resins. Drawbacks of using ion exchange methods include high costs and the fact that conventional resins remove all other ions, in addition to boron ions, necessitating frequent regeneration with acid and base.
Reverse osmosis (RO) processes also are widely used to treat seawater and brackish water. Despite high removal (>99%) of other ionic species from seawater, the removal of boron by RO has proven challenging. Unlike most elements in seawater, boric acid is nonpolar and uncharged; thus, it interacts very differently with RO membrane materials relative to charged salt ions and polar water molecules, making it more difficult to reject. It is difficult for an RO process to achieve an average boron rejection over 90%, the value typically required in order to produce permeate that meets the provisional WHO boron guidelines. Commercialized membranes, such as those used in seawater reverse osmosis plants, generally achieve an average rejection of less than 90% and sometimes as low as 40% with low-pressure brackish water RO membranes (Kim et al., “Boron Rejection by Reverse Osmosis Membranes: National Reconnaissance and Mechanism Study,” Desalination and Water Purification Research and Development Program Report No. 127, July 2009). Thus, the ability of thin film composite (TFC) membranes to reject boron, particularly in the uncharged boric acid form, is often too low for many applications.
Approaches to addressing this problem include chemical treatment of the feed stream and/or the use of two or more filtration steps (e.g., multiple-pass RO), such as the systems described in U.S. Pat. No. 6,709,590. These approaches have drawbacks, as water costs are significantly higher than for single-pass RO systems and some are augmented with boron-specific ion exchange resin for the purpose of converting non-ionized boric acid to negatively charged borate ion (which is more highly rejected), further increasing costs.
Another approach to reducing the boron content in water involves coating the membrane after formation with an agent that enhances boron rejection. Organic biocidal compounds, such as biguanides, particularly polyhexamethylene biguanide (PHMB) or PHMB hydrochloride, have been used to coat reverse osmosis membranes to enhance boron rejection. Such coatings, for example, those disclosed in U.S. Pat. No. 7,491,334, involve treating a polyamide RO membrane after it has been formed by contacting the RO membrane with a chemical solution that contains PHMB.
A disadvantage of the PHMB coating is that the selection of cleaning and treatment options often necessary to maintain proper operation of the membrane is limited. During the course of normal operation, the membranes of RO systems can become fouled by suspended solids, microorganisms, and mineral scale that build up during operation and are deposited on the membranes, causing loss in water output, salt rejection, or both. The membrane elements must be cleaned regularly to extend membrane life and to minimize loss of performance. Typically, reverse osmosis membranes are cleaned first at pH 12, or as high as pH 13, with a sodium hydroxide-based cleaner to remove organic fouling. This step is typically followed by a pH 2, or as low as pH 1, cleaning with citric acid or hydrochloric acid to remove scale.
To preserve the coating, membranes with a biguanide coating can only safely be cleaned with reagents with a pH as high as 11 and as low as pH 4, resulting in increased foulants and decreased performance. Thus, while performance of membranes post-treated with a boron rejection-enhancing agent is good initially, these membranes are not stable to the aggressive cleaning regimens that are typically used involving reagents with a pH higher than 12 and lower than 4. As a consequence, boron concentration levels tend to increase (and boron rejection values tend to decrease) as the coated RO membranes age.
Thus, there remains a need to develop TFC membranes, such as RO membranes, that achieve high boron rejection and remain stable over time, particularly to cleaning regimens, such as regimens that include reagents with high and low pH values, e.g., reagents having a pH of 12 or higher and/or 4 or lower. Also desired are methods for making the membranes. Accordingly, it is among the objects herein to provide TFC membranes, including RO membranes, that achieve high boron rejection values and remain stable over time, particularly after exposure to cleaning regimens that utilize reagents with high and low pH values, and methods for making the membranes.